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Some common logical fallacies

In my discussions with believers, I often hear believers both committing logical fallacies and accusing non-believers of committing them. So for this post, I just wanted to list some common fallacies and how they are used and misused, drawing from examples I've frequently encountered.

1. Ad hominem

Ad hominem is "attacking the man". But it's often confused with insults, like follows:

Person 1: "Blah blah blah"

Person 2: "You're an idiot"

Person 1: "Oh, there you go with making ad hominem attacks

An insult is not the same thing as an ad hominem attack. The proper fallacy occurs when you dismiss an argument because of your value judgment on the person. For example:

"You're wrong because you're an idiot"

"Horatio can't be trusted, so I wouldn't believe his argument"

It's not a fallacy to say that someone is untrustworthy, stupid, or whatever. It's only a fallacy when you conclude that their argument is invalid because they are untrustworthy, stupid, or whatever. It's a fallacy because just as smart and honest people can be wrong, stupid and dishonest people can be right.

2. Genetic fallacy

I hear this one from believers a lot, most commonly misattributing it to statements like, "The main reason you're a Christian instead of a Buddhist is because you were raised in a predominately Christian culture". This may be an erroneous statement depending on the believer, but it's a logically valid proposition – people do tend to adopt the prominent religion of their culture, though of course not all of them do.

Richard Dawkins and John Loftus have often talked about the powerful familial and sociocultural transmission of religious beliefs (it's the basis for Loftus' "Outsider Test for Faith"), but this is only meant to spur critical thinking in the believer, not to disprove the tenets of Christianity. The genetic fallacy would say, "Because Christianity is most commonly transmitted through familial or sociocultural tradition, its tenets are not true." A proposition can be true regardless of how people come to believe it.

3. Argument from ignorance

This is far and away the most common kind of argument that I encounter from believers. Just today I was reading a lecture from Christian theologian Alister McGrath called "The Mystery of the Constants of Nature" which, in a most long-winded way, asserted that because we do not know why the universe has certain constants, we ought to conclude that God is the best explanation.

An argument from ignorance takes many forms, but its most basic is the assertion that a proposition is true because no alternative proposition has been proved. For example:

Science cannot explain why nature has the constants that it does, therefor the best explanation is that God designed the universe.

Science cannot explain how life arose from inorganic matter, therefor the best explanation is that God intervened in the universe to create it.

The fallacy lies in the presumption that science in principle cannot explain these things, or (in the case of the latter) identify them as invalid questions ("meaning" is an abstraction, not an objective thing). Just because we don't understand a great mystery does not mean we should accept unscientific explanations. All valid knowledge must be attained through a methodological process that can objectively identify (and discard) erroneous information, and the only such process we have is science. As soon as believers can develop a methodology for discerning between accurate and inaccurate supernatural claims, I'll be all ears. Until then, the supernatural lies beyond our epistemic bounds, thus natural explanations are more parsimonious. And, given the excellent track record science has of closing gaps in our knowledge, they tend to be much more plausible as well – since even unproven theories can be rooted in observable data.

The argument from ignorance takes other forms, such as these:

Argument from incredulity, in which a proposition is stated as invalid because someone finds it absurd: "I just cannot believe that life can arise from blind physical processes – it had to be designed".

Shifting the burden of proof, in which one asserts that the skeptic must disprove the affirmative claim: "Well, I may not be able to prove that God designed the universe, but you can't prove that God didn't design the universe."

Shifting the goalpost, in which one arbitrarily moves the criteria of "proof" so that it is always outside our boundary of possible knowledge. The most common example I see is that of Intelligent Design advocates suggesting that the transition of life from simple to highly complex forms – a process that takes millions of years in nature – must be observed in laboratory conditions before Intelligent Design can be disregarded.

5. Special pleading

Special Pleading occurs when someone tries to justify a claim as being exempt from well-established logical principles, without justifying the exemption. It's very subtle, but I encounter this one frequently with regard to religious experiences and Biblical history. For example:

"If you do not believe in the historical evidence for the death and resurrection of Christ, you ought to disregard the historical evidence for George Washington." The special pleading in this case is assuming that we ought not to be any more skeptical of supernatural historical claims than we should be about mundane historical claims.

"The real proof to me that God is real is that I have experienced His presence." This special pleading fallacy assumes that one's subjective experiences constitute valid objective knowledge, when they may be tainted by a variety of assumptions and biases. Indeed the entire spectrum of scientific inquiry isn't designed to eliminate bias from the researchers, but methodologically account for the fact that we are all highly biased so that invalid conclusions can be identified and disregarded.

Five should do for now. I'll do a "part 2" down the road and visit a few more.